setClassUnion {methods} | R Documentation

## Classes Defined as the Union of Other Classes

### Description

A class may be defined as the union of other classes; that is, as a virtual class defined as a superclass of several other classes. Class unions are useful in method signatures or as slots in other classes, when we want to allow one of several classes to be supplied.

### Usage

    setClassUnion(name, members, where)
    isClassUnion(Class)

### Arguments

`name` | the name for the new union class.
`members` | the classes that should be members of this union.
`where` | where to save the new class definition; by default, the environment of the package in which the `setClassUnion` call appears, or the global environment if called outside of the source of a package.
`Class` | the name or definition of a class.

### Details

The classes in `members` must be defined before creating the union. However, members can be added later on to an existing union, as shown in the example below. Class unions can be members of other class unions.

Class unions are the only way to create a class that is extended by a class whose definition is sealed (for example, the basic datatypes or other classes defined in the base or methods package in R are sealed). You cannot say `setIs("function", "other")` unless `"other"` is a class union. In general, a `setIs` call of this form changes the definition of the first class mentioned (adding `"other"` to the list of superclasses contained in the definition of `"function"`).

Class unions get around this by not modifying the first class definition, relying instead on storing information in the subclasses slot of the class union. In order for this technique to work, the internal computations for expressions such as `extends(class1, class2)` work differently for class unions than for regular classes; specifically, they test whether any class is in common between the superclasses of `class1` and the subclasses of `class2`.

The different behavior for class unions is made possible because the class definition object for class unions has itself a special class, `"ClassUnionRepresentation"`, an extension of `"classRepresentation"` (see classRepresentation-class).

### References

The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book Programming with Data (John M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

While the programming interface for the methods package follows the reference, the R software is an original implementation, so details in the reference that reflect the S4 implementation may appear differently in R. Also, there are extensions to the programming interface developed more recently than the reference. For a discussion of details and ongoing development, see the web page http://developer.r-project.org/methodsPackage.html and the pointers from that page.

### Examples

    ## a class for either numeric or logical data
    setClassUnion("maybeNumber", c("numeric", "logical"))

    ## use the union as the data part of another class
    setClass("withId", representation("maybeNumber", id = "character"))

    w1 <- new("withId", 1:10, id = "test 1")
    w2 <- new("withId", sqrt(w1)%%1 < .01, id = "Perfect squares")

    ## add class "complex" to the union "maybeNumber"
    setIs("complex", "maybeNumber")

    w3 <- new("withId", complex(real = 1:10, imaginary = sqrt(1:10)))

    ## a class union containing the existing class  union "OptionalFunction"
    setClassUnion("maybeCode",
        c("expression", "language", "OptionalFunction"))

    is(quote(sqrt(1:10)), "maybeCode")  ## TRUE

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